High-voltage windings are used in various dynamoelectric machines, such as motors or generators. For example, high-voltage windings commonly referred to as stator windings are used in high-voltage electrical generators. A high-voltage winding, such as a stator winding, can be formed from at least one winding bar that, in turn, comprises one or more electrical conductors. The electrical conductors individually are formed of a highly conductive material, such as copper. The electrical conductors are ordinarily individually-insulated and bundled together to form the winding bar. The bundle, in turn, is surrounded by insulation, often referred to as a winding insulator or groundwall insulator. The groundwall insulator can be a single-sided epoxy resin/mica paper tape wrapping, usually comprising multiple layers of a glass-backed mica-tape.
Overlaying the groundwall is an outer conductive ground electrode that surrounds the groundwall insulator. The outer conductive ground electrode can be a coating of conductive paint or a wrapped conductive tape over the groundwall insulator. The outer conductive ground electrode is connected to ground so that the voltage of the outer surface of the high-voltage winding is at ground potential.
The tape may be applied half lapped, abutted or in any other suitable manner. Generally, multiple layers of the mica tape are wrapped about the coil with sixteen or more layers generally being used for high voltage coils. The number of layers may be decreased depending on the power of the generator and the effectiveness of the insulator in both its abilities to insulate electrically and conduct heat. To impart better abrasion resistance and to secure a tighter insulation, a wrapping of an outer tape of a tough fibrous material, for example, glass fiber, asbestos or the like may be applied to the coil.
Therefore, what is referred to as insulating tape is actually composed of multiple layers of tape that have different properties. The inner-most layer is referred to as the groundwall insulation. Wrapped around this is the conductive layer. The conductive layer provides a low resistance and doesn't allow voltage to be present between the outer coil surface and the core.
The insulating tape is generally impregnated with a resin to improve many of its overall properties. There are many methods of coating materials with epoxy resins and then curing the product, One such method is vacuum pressure impregnation (VPI). This method is used on devices such as stator conductor coils. A mica/glass insulating tape is applied to the coils, then the coils are placed in a vacuum vessel and a vacuum is applied. After a period of time, resin is admitted to impregnate the coils. Pressure is applied to force the resin in and minimize voids, which will affect conductivity. After this is completed, the coils are heated to cure the resin. A variation of this, global VPI (GVPI) involves the process where dry insulated coils are wound, and then the whole stator is vacuum pressure impregnated rather than the individual coils.
If the conductor is not secure against the generator assembly, electric discharge will result. This adversely affects the performance of the machinery, and also causes cumulative damage to the generator, conductor and insulation tape. In order to prevent such a discharge, the conductive layer of the insulating tape itself is typically formed of at least two layers, which are referred to as the outer conductive layer and the inner conductive layer. The outer layer of conductive tape will be in firm contact with the generator core, while the inner conductive layer will be in firm contact with the groundwall insulation. This, however, creates a problem, since the conductor and the generator core often have minor movements independent of one another due to such things as heating and vibration. This is referred to as a difference of movement. If the outer conductive layer of the insulating tape is in firm contact with the generator assembly, and the conductor moves independently of the assembly, stresses are created on the insulating tape.
These stresses may cause the tape to tear, ruining the insulation around the conductor coil. One solution to prevent this comprises providing a slip layer in the insulating tape to provide mechanical isolation between the generator assembly and the electrical conductor. This slip layer is sandwiched between the inner conductive layer, which is in contact with the conductor, and the outer conductive layer, which is in contact with the generator assembly. The slip layer may consist of a mica-filled tape that is interwoven with a conductive tape. The mica-filled slip layer consists primarily of relatively large mica flakes, i.e., mica splittings, typically provided in a mica splittings tape. The large mica flakes are generally larger than those used in other insulating layers. Therefore they are not well bonded together and can slip relative to each other. This slip layer allows for a minor difference of movement between the inner and outer conductive layers, without causing any tears or damage to the tape. In addition, the slip layer also aids in stator coil removal from a wound GVPI stator winding.
This solution, however, is not without its own problems. One concern is that the slip layer, because the mica-filled tape has large flakes, it is delicate and therefore is more susceptible to handling damage. This means that the slip layer, and often the entire insulating layer, has to be wound around the conductor by hand, rather than using more efficient machinery. Also, the amount of slip that the slip layer allows for provides only a moderate difference of movement.
In addition, the current installation of mica splittings tapes provided in GVPI processes is not provided in such a way as to produce dielectric properties since the mica tape is typically applied with a butt joint pattern over the insulated stator coil conductor wire stack in order to allow for easier GVPI resin impregnation. Further, current high quality mica splittings tapes required for high voltage applications are relatively expensive and are not commonly available.
Accordingly, there is a need for a mechanical isolation layer providing a stress reducing interface between an electrical conductor and a generator core that is readily manufactured and is capable of high voltage applications.